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BASIC NEC 2023

Transformer Practice Quiz Test-1

Free online transformer practice quiz. 25 basic electrical questions on turns ratio, EMF equation, transformer losses, three-phase connections, auto-transformers, voltage regulation, and transformer testing. Good for electrician exam prep.

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A transformer transfers electrical energy between two windings using a changing magnetic field, with no moving parts. Key basics: the turns ratio sets the voltage ratio between primary and secondary (V_s/V_p = N_s/N_p). A transformer only works on AC supply because it needs a changing flux to induce voltage. Core losses are split into hysteresis loss (from the core material) and eddy current loss (reduced by laminating the core). Three single-phase transformers, or specially wound three-phase units, can supply three-phase power using connections like star-delta, delta-star, or open-delta.

25

Questions

38 min

Time

70%

Pass

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Transformer Practice Quiz Test-1 — Question List

Question 1: What happens when a DC supply is supplied to Primary of a transformer?

A small current flows in primary
Core losses will increase
Leakage reactance is increased
Primary winding may burn out

Question 2: <span style="color: #000000;">A transformer has an efficiency of 90% which works at 120 V, 3.6 kW. If the secondary voltage is 240 Volts, what is the primary current? </span>

40 amperes
20 amperes
30 amperes
50 amperes

Question 3: A transformer having 1500 turns in primary winding 300 turns in secondary winding is supplied with 120 volts AC. What is its secondary Voltage?

24 Volts
48 Volts
12 Volts
60 Volts

Question 4: A three-phase 4 wire electrical system phase-phase voltage is 480 volts. what is its phase to neutral voltage?

377 volts
277 volts
480 volts
240 volts

Question 5: The Scott connection is used to change three phase power into two phase power.

True
False

Question 6: What is the phase angle between primary and secondary of a transformer?

180 degrees
90 degrees
270 degrees
360 degrees

Question 7: The ampere current in primary side of a transformer is directly proportional to ampere current in secondary.

True
False

Question 8: What is the best way to connect to single phase transformers to act as three phase transformer?

open-delta
Scott connection
deltas-star
none

Question 9: Transformer is laminated to reduce .....................

eddy current losses
hysteresis losses
both eddy current and hysteresis losses
None

Question 10: The Transformer is not used in DC Power supply system because

high losses will occur
There is no need of step up of dc power supply
the rate of change of flux is zero

Question 11: A step-down transformer reduces the secondary voltage compared to the primary. What happens to the secondary current compared to the primary current (ignoring losses)?

Secondary current increases
Secondary current decreases
Secondary current stays the same
Secondary current becomes zero

Question 12: The no-load current drawn by a transformer primary has two components: the magnetizing component and the:

Core loss (working) component
Short circuit component
Leakage component
Overload component

Question 13: An auto-transformer is different from a normal two-winding transformer because it has:

Two completely separate windings
Only one winding, shared between primary and secondary
No iron core at all
A rotating armature

Question 14: In a three-phase STAR-DELTA connected transformer bank, the star (wye) side is normally used for:

The high-voltage side, so a neutral point is available
The low-voltage side only, and never the high side
Only for single-phase transformers
It has no particular benefit over delta-delta

Question 15: Voltage regulation of a transformer is best described as:

The change in secondary voltage from no-load to full-load, as a percentage
The ratio of primary to secondary turns
The efficiency of the transformer at full load
The temperature rise of the winding under load

Question 16: "All-day efficiency" of a distribution transformer is calculated using:

Energy output over 24 hours divided by energy input over 24 hours
Power output at rated load divided by power input at rated load
Peak voltage divided by peak current
Maximum current divided by rated current

Question 17: ONAN is a common cooling method used for large oil-filled power transformers. What does ONAN stand for?

Oil Natural, Air Natural
Oil Natural, Air Forced
Oil Forced, Air Natural
Oil Forced, Air Forced

Question 18: A tap changer on a power transformer is used to:

Adjust the turns ratio to control output voltage
Change the frequency of the output
Increase the number of phases
Cool the transformer oil faster

Question 19: Why should the secondary of a current transformer (CT) never be left open while the primary is carrying current?

The CT will simply stop working with no other effect
A high, dangerous voltage will appear across the open secondary
The primary current will drop to zero
The CT ratio will automatically change

Question 20: A potential transformer (PT / voltage transformer) is mainly used to:

Step down high system voltage to a safe level for meters and relays
Step up the current for motor starting
Convert AC supply into DC supply
Store energy for backup power

Question 21: Before two single-phase transformers can be safely connected in parallel to share a load, they must have:

The same voltage ratio and matching polarity
The same physical size and weight
The same oil color and tank shape
Different frequencies for load sharing

Question 22: A Buchholz relay is a protective device fitted to:

Oil-filled transformers, between the tank and the conservator
Dry-type transformers only
DC generators only
Induction motors only

Question 23: The Sumpner's test (back-to-back test) on transformers is mainly used to find:

Both iron and copper losses with full-load heating, using two identical transformers
Only the turns ratio of a single transformer
Only the insulation resistance of the winding
The speed of the cooling fan motor

Question 24: A single-phase transformer has 2400 primary turns and 240 secondary turns. If the primary is supplied at 480 V, what is the secondary voltage?

4.8 V
48 V
240 V
480 V

Question 25: Two single-phase transformers connected in an open-delta (V-V) bank can supply what percentage of the three-phase load that three transformers in a full delta bank could supply?

About 58% (57.7%)
Exactly 66.7%
100%, same as a full delta bank
33.3%

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 (NFPA 70)